Transcript Valence Bond and MO Theory

Bonding III
Valence Bond Theory
and
Molecular Orbital Theory
Valence Bond Theory
• Extends the Lewis concept of electron pair
bonds by introducing the notion of orbital
overlap and hybridization.
• Hybridization is necessary to account for
molecular geometry predicted by VSEPR
• Hybrid atomic orbitals are constructed by
making linear combinations (sums and
differences) between hydrogen-like
orbitals in the valence shell of each atom
Hybridization
• Example: BeH2
• Linear Molecule, equivalent Be-H bonds
• Symmetry Adapted Linear Combination (SALC)
of hydrogen-like atomic orbitals in the valence
shell of Be
• Basis set consists of: 2s, 2px, 2py, 2pz
• Only 2s and 2px are mixed; 2py and 2pz retain
their original form.
• “ideal” hybrids ½ s, ½ p character
• Energetic “cost” of hybridization (promotion
energy) is offset by formation of stronger bonds
sp hybridization
Bonding Description
• Valence electrons of each atom are
distributed among the available orbitals.
• In this case, the two valence electrons of
Be are placed in the sp hybrid orbitals; the
remaining p orbitals are vacant.
• Each H atom has one electron in the 1s
orbital.
• Orbitals of each atom overlap to form
electron pair bonds.
Sigma and Pi Bonding
sp3 hybridization
VBT description of H2O
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Oxygen has sp3 hybridization
Ideal tetrahedral angle is 109.5o
Observed bond angle in H2O is ~ 105o
This indicates that hybrids involved in O-H
bonding orbitals have more p-character
• Lone-pair orbitals on oxygen have more scharacter
Molecular Orbital Theory
• Fundamentally different than VBT
• Valence atomic orbitals of all atoms in the
molecule form the basis set
• For convenience, orbitals on symmetry-related
atoms are combined into symmetry-adapted
linear combinations called ligand group
orbitals (LGOs)
• Atomic orbitals on central atom are mixed with
LGOs to generate molecular orbitals (MOs)
• In the final step, valence electrons are placed
into MOs starting at lowest energy
Molecular Orbitals of H2
• Basis set of orbitals consists of only the 1s
orbital on each H-atom
• Linear combinations of these orbitals
produce two molecular orbitals
• + combination gives bonding orbital
s(1s) (constructive interference between
wavefunctions
• - combination gives antibonding orbital
s*(1s) (destructive interference)
Molecular Orbitals of H2
Molecular Orbitals of Li2
NOTE: Both bonding and antibonding orbitals arising from the 1s
orbitals are filled – no net bonding results from core electrons!
p-Orbital overlap modes
Homonuclear Diatomic with p-Orbitals
Effect of s-p interaction
For atoms early in the 2nd period, energy separation between 2s and 2p
orbitals is small. This allows mixing of s(2s) and s(2p) orbitals that
raises the energy of the s(2p) orbital above that of the p(2p)
MO diagram for O2 and F2
Later in the period, the energy separation of the 2s and 2p orbitals
Is greater (decreasing s-p interaction), so s(2p) drops below p(2p)
Molecular Orbital diagrams
for 2nd row diatomics
Heteronuclear Diatomics
Strength of orbital interactions depends upon:
• Symmetry match
• Spatial overlap
• Energy overlap
MO diagram for HF
Bonding in H2O
Polyatomic Molecules
• Group orbital approach
• Symmetry-Adapted Linear Combinations
• Examples: H2O
Character Table for C2v
MO’s of H2O (A1 symmetry)
O(2s) and O(2pz) AO’s mix with H(f1) LGO to form 3 MO’s:
y(1a1) = (s) + (f1)
y(2a1) = (pz) - (f1)
Strongly Bonding
Weakly Bonding
Y(3a1) = (pz) + (f1)
Strongly Antibonding
MO’s of H2O (B2 symmetry)
O(2py) AO mixes with H(f2) LGO to form 2 MO’s:
y(1b2) = (py) + (f2)
y(2b2) = (py) - (f2)
Bonding
Antibonding
Molecular Orbitals of Water
Bonding in SF6
Fluorine SALCs for SF6
MO diagram for SF6